Solid oxide fuel cells (SOFC) typically operate at temperatures in excess of 800° C. Elevated temperatures increase catalytic reaction rates and ion transport through a solid electrolyte in the fuel cell. Typical fuel cells are heated by an external heater that heats the fuel to a temperature sufficient for catalysis. The heat from the exothermic reaction further increases the cell's operating temperature to an optimal level. However, the time required for sufficient amounts of heated fuel and air to pass through the fuel cell stack and heat the cell elements to a level where the catalytic reactions are self-sustaining reduce the efficiency of the cell and waste fuel. As a result, it is desirable to have a more efficient method of heating the fuel cell stack.
Fuel cells are produced with both dual-chamber and single-chamber designs. Air and fuel are introduced to a dual-chamber system separately. In the dual-chamber design, the cathode is exposed only to air, and the anode is exposed only to fuel. The electrolyte is gas-tight, only permitting oxygen ions, not electrons, to pass through. As fuel cells become smaller, the electrolyte membrane becomes thinner, decreasing the resistance for the transfer of oxygen ions from the cathode to the anode. However, thinner membranes also exhibit decreased mechanical stability. They are also more difficult and expensive to manufacture, and the necessity for a gas-tight electrolyte further increases the complexity and expense of the dual-chamber design.
Single-chamber fuel cells eliminate some of these problems. Fuel and air are introduced to both the anode and the cathode surfaces as a mixture, obviating a gas-impermeable electrolyte membrane (Hibino, Science, 2000, 288:2031). However, enabling use of a fuel-air mixture does not solve the mechanical difficulties of the single-chamber device. The need for mechanical robustness decreases the available surface area of the electrolyte and catalysts, further decreasing the power output per unit area. In addition, it is difficult to reduce fuel usage or system temperature during low power demand without reducing system efficiency. As a result, it is desirable to have a fuel cell design that increases catalytic surface area while maintaining mechanical stability.